netdev - Re: [PATCH] macvlan: lockless tx path

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Date:	Wed, 10 Nov 2010 19:18:40 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	David Miller <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] macvlan: lockless tx path

Le mercredi 10 novembre 2010 à 09:53 -0800, Ben Greear a écrit :

> I agree, but if these can be read from user-space, it can be tricky to make
> solid code to deal with wraps when the thing wrapping can be 32 or 64 bits,
> depending on whether the kernel is compiled 32-bit or 64-bit.
> 
> So, my preference is to use u32 or u64 so there is no guesswork involved.
> 
> To be sure, this problem exists in lots of places already (/proc/net/dev comes to mind),
> but the fewer places the better in my opinion.
> 

On a 32bit kernel, very few devices provide 64bit counters, so an
application reading /proc/net/dev should be prepared to handle 32 or
64bit counter.

On a 64bit kernel, many devices still provide 32, 36, 40 bit counters
(hardware based). Same conclusion for userspace.

So really, an SNMP application must be able to cope with any counter
width.

As percpu data is going to hurt in the 4096 cpu cases, we should try to
not make percpu structures too big.

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